1) Field of the Invention
This invention relates to an optical transmission system, a dispersion compensation method for an optical transmission system and a simulation equipments suitable for use when dispersion compensation on an optical transmission line is performed.
2) Description of the Related Art
In recent years, development for practical use of an optical transmitter for increase of the capacity and the transmission distance of an optical transmission system is demanded. Particularly, there is growing expectation to introduce an optical transmitter which adopts an optical modulation method suitable for increase of the capacity and the transmission distance into an actual system. In order to satisfy the expectation, introduction of an optical transmission system for which a differential phase shift keying method such as DPSK (Differential Phase Shift Keying) or DQPSK (Differential Quadrature Phase Shift Keying) is used is anticipated.
Further, in an optical transmission system, a design for chromatic dispersion compensation is significant. Depending upon achromatic dispersion compensation method, there is the possibility that non-linear effect of light in an optical fiber may cause significant deterioration of the transmission quality. Particularly, in a 40 Gbit/s WDM (Wavelength Division Multiplexing) transmission system, since the withstanding property against chromatic dispersion is poor, it is necessary to perform appropriate dispersion compensation design.
It has been examined to design an NRZ (Non Return to Zero) modulation method which is one of conventional modulation methods such that the relationship between a pre-line dispersion compensation amount (dispersion compensation amount on the transmitter side) and an in-line dispersion compensation ratio (proportion of dispersion compensation amount for a chromatic dispersion of transmission line in each repeater) is kept at a constant value irrespective of the number of transmission spans, as disclosed, for example, in Yann Frignac and Jean-Christophe, “Numerical optimization of pre- and in-line dispersion compensation in dispersion-managed systems at 40 Gbit/s”, International Conference OFC 2002, article No. ThFF5, held March, 2002, pp. 612-613 (hereinafter referred to as Non-Patent Document 1) or Japanese Patent Laid-Open No. 2005-295126 (hereinafter referred to as Patent Document 1).
However, in an optical transmission system to which a differential phase shift keying method such as DPSK or DQPSK is applied, even if a method of compensating for chromatic dispersion which has been applied to such an NRZ modulation method as described above is used, it is difficult to achieve sufficiently good quality of a signal to be reproduced at a receiver side. Accordingly, in the optical transmission system which adopts a differential phase shift keying method, it is required to perform setting of the pre-line dispersion compensation amount or the in-line dispersion compensation amount from a point of view different from that of the conventional technique.
In this regard, it is expected that setting of an optimum dispersion compensation amount is searched by randomly combining various variations regarding the pre-line dispersion compensation amount and the in-line dispersion compensation amount in each repeater to measure and evaluate the signal quality at a receiver side. However, such a searching method as just described gives a subject that much time is required for a process for specifying setting of an optimum dispersion compensation amount.